Porch fireplace chase going thru conditioned attic space
New build, zone 2. Plans call for a prefab’ed fireplace on the back screened porch, i.e., outside the building envelope. To be clear, the FP opening will be accessible from the porch only. The chimney chase will go through the house’s attic. The attic above the FP is unvented, part of the building envelope, and will probably have OC spray foam insulation on the underside of the roof sheathing. The FP “enclosure” below the attic will not be insulated since it is outside the building envelope.
Typically in this area (where attics in 99% of houses aren’t in the building envelope and insulation is blown in on the attic floor instead of the underside of the roof), the FP enclosure stops at the ceiling, the flue pipe is exposed in the attic, and an enclosed, uninsulated chase is built from the roof up for the outside flue pipe.
In order to keep the attic relatively tight and insulated as part of the building envelope, would it suffice to frame and sheath a flue chase from the attic floor to the roof, then either spray foam the outside of the sheathing or attach rockwool batts to the outside of the chase within the attic?
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Just to help me envisage this: The main attic is conditioned and insulated at the roof. The attached porch will be part of the conditioned attic, and insulated at the roof and the ceiling. My assumption is that you are doing that so you can enclose and condition the porch if y0u choose to later.
A chase makes lots of sense. It eliminates worries about air-sealing the flue as it penetrates the roof and ceiling, simplifying the installation a lot.
Fireplaces on porches are great!
You're pretty close. I added a new post on Oct 10 with subject "Insulation for Pretty Good House Goals" that has a cross section of the house and shows the screened porch.
The main reason the attic over the porch is insulated and part of the envelope is because a lot of the HVAC ductwork is run in that part of the attic to allow the attic to be finished in the future without having to redo a lot of ductwork. Although now that you mention it, possibly enclosing the screen porch in the future sounds like a good idea, too.